Electronic warfare systems are used on modern military aircraft as part of their offensive and defensive capabilities. These electronic warfare systems emit RF signals that travel through space. Radar systems use RF emissions to locate and track opposing aircraft and some radar systems are incorporated within missiles to assist in the self-guided propulsion of a missile to its target. An electronic warfare search receiver is used defensively to detect those RF emissions. The receiver searches the range of frequencies (the RF spectrum) in which the RF emissions are likely to occur. The receiver then detects and analyzes the nature of the RF signals. By determining the characteristics of the signals received, the defender will know the nature of the threat and, for example, will know if a radar guided missile has "locked on" to the defenders aircraft. These systems are used in friendly as well as unfriendly aircraft. In a tactical or strategic environment, the number of aircraft and the density and diversity of the emissions in the RF spectrum is quite large and is expected to increase. Existing detection and monitoring equipment that use wide band search receivers will find the RF emissions difficult or impossible to successfully monitor in such an environment. For example, some existing wide band receiver designs employ a threshold detector that requires the incoming signal to attain a certain amplitude before it is recognized as a true signal apart from the ordinary RF background noise. These receivers are incapable of detecting pulse-on-pulse conditions and accurately reporting the parameters of both pulses. These receivers are also not capable of detecting pulses that occur near the trailing edge of a first pulse. In this circumstance, the trailing edge of a first pulse does not cross back over the threshold level before the occurrence of the second pulse. With the existing designs, it is entirely possible that a first RF pulse received will effectively prevent detection of a second RF pulse, from another emitter, either during the presence of the first pulse or immediately after the first pulse. The first emission source may be identified but the second source is, in effect, masked.
It is unlikely that a single receiver type will be capable of meeting all offensive or defensive threat detection and analysis requirements dictated by the future electronic warfare environment. Instead a set of search and analysis receivers of complimentary capabilities are likely to be required to meet future demands. Trade offs between probability of intercept, bandwidth, simultaneous signal resolution, sensitivity, receiver complexity and power consumption are necessary. Detecting pulse-on-pulse conditions and trailing edge pulse conditions as well as accurately reporting the parameters of both pulses in a pulse-on-pulse condition are important abilities for a modern receiver.
In traditional electronic warfare/electronic support measures (EW/ESM) receivers, techniques are used in which the receivers video output is digitized. Digitization occurs when the amplitude of the input signal exceeds a predetermined threshold level. After the threshold has been crossed, the signal parameters for that signal are digitized. However, if a second signal occurs before the first signal drops below the threshold, then the second signal will not be detected. This allows a continuous wave or long pulsed width signal to prevent detection of subsequent signals occurring simultaneously with the first signal, even if the subsequent signals are significantly larger in amplitude than the first signal.
Simultaneous signal resolution is an important requirement that needs to be addressed in order to reduce the risk that an enemy radar signal will go undetected. With this background it is desirable to provide a wide band receiver that has a high probability of detecting simultaneous and trailing edge signals and that can also measure the phase, frequency, time of arrival, pulse modulation, pulse width and amplitude of each signal.